Today, radiation therapies for cancers are rapidly developing centering around various irradiation methods, and along with the development, the importance of measurement of three-dimensional absorbed dose is increasing. To evaluate radiation absorbed by the living body, it is necessary to use a dosimeter sensor having the same effective atomic number as that of biological tissues. Dose measured with a sensor having a different effective atomic number cannot be used to measure dose absorbed by biological tissues accurately.
A two-dimensional dose distribution is now obtained by Gafchromic film or imaging plate (IP) photoreceptor. However, since Gafchromic film can be used only once, in-plane sensitivity coefficient cannot be obtained, nor can disrupted images resulting from uneven coating of photoreceptors be corrected. Hence, Gafchromic film has problems in quantitative capability. Further, Gafchromic film has a small dynamic range and this problem imposes many restrictions on use of the film. Meanwhile, since IPs are not biological tissue equivalent, it is virtually impossible to apply IPs to three-dimensional measurement. A method of measuring three-dimensional dose distribution using a molded product of a polymer gel in which a biological tissue-equivalent fluorescent substance is dispersed is also being studied, but the method is highly burdensome in terms of facilities and labor and is not practical.
As a thermoluminescent substance, a copper-containing lithium triborate (Non-patent Document 1) is known. The non-patent document discloses that manganese-containing lithium triborate acts as a thermoluminescent substance, but the document does not refer to a specific synthetic method or characteristics and hence, manganese-containing lithium triborate cannot be produced based on the document. Further, it is reported that pure lithium triborate crystals containing no additives exhibit thermoluminescence (Non-patent Document 2), but the document does not refer to manganese-containing lithium triborate.